Gravity-induced loss of consciousness (“GLOC”) is a prototypical example of a phenomenon of reduced cerebral blood flow that occurs when someone is subjected to substantially increased gravitational loads (+Gz) for a sustained period. High-performance aircraft, such as fighters, allow maneuvers that generate +Gz that exceed the limits of the human body. This predisposes to GLOC and a serious degrading of physiological and cognitive performance. GLOC is one of the primary physiological threats to pilots and crews of high-performance aircraft. Since the mid 1980s, one branch of the US military, the United States Air Force, has lost 29 aircraft and 22 pilots to GLOC. (The Effect of Negative Gz Recovery from GLOC on Cerebral Oximetry, Broughton, presentation at USAF School of Aerospace Medicine, Brooks AFB, Texas (2003).) Similar loss rates can be expected for the other services flying high performance aircraft. In addition to the loss of life, the cost of training and lost aircraft is staggering.
Almost loss of consciousness (ALOC) is even more common than GLOC. Symptoms include euphoria, apathy, displacement, depersonalization, poor response to auditory stimuli, immediate memory difficulties, sensory abnormalities, motor abnormalities, confusion, and dream-like state without loss of consciousness, which are considered precursors of GLOC, which is defined as “A state of altered perception wherein one's awareness of reality is absent as a result of sudden, critical reduction of critical blood circulation caused by increased G forces”. (Morrissette K L, McGowan D G. Further support for the concept of a G-LOC syndrome: a survey of military high-performance aviators. Aviat Space Environ Med. 2000; 71:496-500.; Burton RR, G-Induced Loss of Consciousness: Definition, History, Current Status. Aviat Space Environ Med. 1988; 59:2-5.)
Some methods have been developed to increase G-level tolerances, including centrifuge training, weight training, the anti-G suit, positive pressure breathing, anti-G straining maneuvers and postural modification in the cockpit. The current capabilities of trained individuals to maintain clear vision during sustained exposures to +9 Gz, an increase in protected +Gz tolerance of about +4 Gz over World War II fighter pilots, is largely a result of combined use of a G suit and self-protective straining maneuvers such as the M-1, L-1 and pressure breathing, all of which are variants of the Valsalva maneuver developed in the 1940s. (G-induced Loss of Consciousness and its Prevention, Earl Wood, (1988) Mayo Clinic, Rochestor, Minn.) However, despite such training, a review of ten fatal crashes attributed to GLOC shows that such measures fall short of addressing the problem. Id. The Wood review notes that the likely causes of such failures were: (1) increased capability of jet-powered fighters to sustain, with minimal pilot effort, accelerations in the 7-10+Gz range for periods longer than the symptom-free 3-8 second cerebral ischemic anoxic period which precedes GLOC, (2) an improperly performed Valsalva-type straining maneuver, and (3) development of a hypotensive vasovagal type reaction.
The inventors believe that currently used techniques do not adequately address the detection of reduced cerebral blood flow. For example, the problem of GLOC (which for the purposes of this document pertains to both ALOC and GLOC) puts the burden on the pilot to realize when he/she is about to sustain GLOC. Further, reduced cerebral blood flow is a serious medical condition which can lead to irreversible brain injury. At present there are no simple and reliable technologies for measuring cerebral blood flow noninvasively. For example, decreased cerebral perfusion may occur during surgery, trauma, sleep disorders, cerebral vascular insufficiency (eg. Ischemic stroke), hypotension from a wide variety of causes or during ventilatory management